What would cause me to have low cold water pressure throughout the entire house but the hot is fine? And how can I fix it?

The first thing we must know is your house pressure. How old is your home and what material are your pipes? Copper, Plastic or Galvanized? Pump or city water?
Do you get full pressure out of your outside hose bibs? Let me know, Tom

I don't know what you mean by house pressure, the house is over 200 yrs old, the pipes are galvanized to copper and I have city water. And I get full pressure outside.

Lets start with basic principals of hydraulics. The pressure for your hot water is created by the pressure of your cold water entering the water heater. So we now know that you do have, must have, good cold pressure at least at your water heater. Somewhere further along the line you must have a restriction in a main cold supply line, a big restriction. Not uncommon in very old galvanized pipes. I would start by replacing as much galvanized supply line as you can afford